Most years, the entire state of Texas reports about 20 cases of mumps, but an outbreak in Johnson County has affected almost 30 people, mainly children, in recent weeks.

"It's really unusual," Johnson County's public health authority, Dr. Alvin Adams, told the Fort Worth Star-Telegram. "We're having more than a year's worth of cases in one community."

Officials have identified 28 cases of mumps in the area, mostly in school-age children in Cleburne and Keene, Adams said.

Health officials think they have traced the virus to an outbreak in Arkansas, where several students from Keene visited family about a month ago. Five people returned home with the virus.

Keene ISD has 1,100 students, and 34 of them may be at risk because they don't have the relevant vaccination, according to KXAS-TV (NBC5).

"They won't be allowed back in school for 26 days, long incubation period for mumps," said Keene ISD Superintendent Ricky Stephens said.

The district's three schools had planned to offer vaccines last week, NBC5 reported.

Mumps is most commonly known for its symptoms of puffy cheeks and a swollen jaw, a result of swollen salivary glands, according to the Centers for Disease Control and Prevention. The condition is often accompanied by a fever, headache, muscle aches, tiredness and a loss of appetite. Most people with mumps recover completely in a few weeks.

Severe complications are rare, but the disease can lead to inflammation in key organs such as the brain, testicles and ovaries, the CDC says.

Mumps spread through saliva or mucus from the mouth, nose or throat of an infected person — through coughing, sneezing or talking, sharing contaminated utensils and touching objects or surfaces with unwashed hands that are then touched by others.

Mumps cases in the country have been on a steady decline in recent years. In 2010, 2,612 instances of the disease were reported to the CDC. In 2012, 229 cases were registered.

There's been a huge drop in how many people have been affected by the disease since the U.S. mumps vaccination program first started in 1967, according to CDC data. At the time, about 186,000 cases were reported each year. Since the pre-vaccine era, the CDC notes, there has been a more than 99 percent decrease in the number of people with mumps in the U.S.